It is a normal feature of these high output power transmitters to include a power amplifier stage prior to the antenna. Further it is common to provide a degree of amplitude control to the output power amplifier to provide adjustment, optimization and control of the transmitter. Switching mode power amplifiers are known from the state of the art as particularly efficient power amplifiers. Class-E amplifiers, for example, which have been presented in U.S. Pat. No. 3,919,656 are switching mode power amplifiers, which can theoretically approach a power efficiency of 100%. Switching mode power amplifiers are used for example in transmitter architectures, which require low power consumption, like transmitter architectures employed for mobile devices.
While a switching mode power amplifier can be very efficient, it is inherently very non-linear, i.e. the amplitude of its output signal is not affected linearly by a change of the amplitude of its input signals within the regular operating range. On the other hand, a switching mode power amplifier does not alter significantly the phase of input phase modulated signals. Thus, switching mode power amplifiers are rather suited for amplifying constant-envelope phase modulated signals than amplitude-modulated signals.
In some cases, signals that are to be amplified have no envelope variation in the first place. In some other cases, the switching mode power amplifier does not see the envelope variation, e.g. in the LINC system (LInear amplification using Non-linear Components) proposed by D. C. Cox of the Bell laboratories in “Linear Amplification with Nonlinear Components”, IEEE Transactions on Communications, COM-22, pp. 1942 to 1945, December 1974, or when using a band pass pulse position modulation (PPM). In the latter case, the structure comprising the switching mode power amplifier as a whole takes care of generating the correct envelope for the transmitter output signal.
A structure employing a switching mode power amplifier is given for example with a conventional envelope elimination and restoration (EER) transmitter. In such an EER transmitter, a constant-envelope phase-modulated radio-frequency signal is input to the switching mode power amplifier for amplification. The envelope is then restored in the switching mode power amplifier by varying its supply power.
In most applications, it is required that the average power level of the signals output by a power amplifier can be controlled, possibly even over a very large dynamic range. In a conventional power control of the output power level of a power amplifier, a variable gain amplifier (VGA) is arranged in front of the power amplifier, which pre-amplifies the input signal according to the desired output power level. Since a switching mode power amplifier is inherently very non-linear, however, a conventional power control is not suitable for a switching mode power amplifier. On the other hand, the dynamic range that can be achieved by varying the power supply to the switching mode power amplifier mentioned above is restricted by a lower limit. This lower limit results from a leakage of an input radio frequency signal through the transistor of the switching mode power amplifier due to its parasitic capacitances.
There are various prior art approaches for dealing with the power control of switching mode power amplifiers; however, none of these approaches provides for a linear power control of a switching mode power amplifier over a very large dynamic range while preserving the efficiency of the amplifier. Further none of these approaches fits well with the desire for exploiting monolithic integration on semiconductors, such as CMOS due to the requirements of absorbing large die area for the integration of the control elements, which further add to the manufacturing and packaging issues and complexities.
It would be advantageous to provide a circuit topology those functions well for integration with CMOS amplifiers that support accurate amplitude control across a dynamic range sufficient to meet existing standards such as GSM and EDGE modulation requirements.